Saruman commands his army from Isengard in The Lord of the Rings: The Two Towers.
Isengard is a fortified ring-shaped complex in the southern end of the Misty Mountains. At its center stands Orthanc, the tall black tower where Saruman studies, plans, and communicates with Sauron.
The fortress becomes a major industrial base during the war. Saruman uses it to breed and equip Uruk-hai, while the surrounding landscape is damaged by furnaces, pits, and extensive tree cutting.
Isengard is not the same place as Orthanc. Isengard is the larger fortress and surrounding territory; Orthanc is the tower within it. The Ents later attack and flood Isengard.